1. Field of the Invention
This invention relates generally to telecommunications systems and, more particularly, to communication apparatus for use in such systems capable of performing data communications at variable communication rates.
2. Description of the Related Art
Currently available mobile radio communications systems include, but not limited to, personal digital cellular (PDC) systems and personal handyphone systems (PHS). In recent years, digitalization has been applied to access schemes for providing access between mobile stations and base stations in such communications systems. This permits the systems to achieve high-speed data communications as well as traditional speech-based or voice telecommunications services.
Telecommunications systems of earlier generations employing as such digital access scheme the time division multiple access (TDMA) technique have been typically designed to perform data communication with a single time slot assigned in a data communication event on a per-event basis. As a need is felt to speed up communication services in recent years, attempts are made to assign multiple time slots to one data communication event.
Regrettably the speed-up approach is encountered with a problem which follows.
A single user exclusively owns or “monopolizes” a plurality of times slots that are limited in number at a base station, which would badly affect other users' communications resulting in a decrease in their useable slot number. This is a bar to retainment of the equity in public communications services.
On the contrary, unconditionally limiting the number of usable time slots during data communication to one per user would result in a decrease in data transfer speed or rate while reducing the use efficiency of system resources due to an increase in number of non-use or “idle” time slots where other users are absent.
One prior known way of solving these conflicting problems is to employ variable time-slot number control techniques, which are for controlling the number of time slots being assigned to a certain user who presently requests high-speed data communication services so that the slot number varies even during communication in accordance with the number of idle time slots at a base station.
Here, an explanation will be given of an event of data communication between a mobile station PSa of PHS and a fixed station FS on the integrated services digital network (ISDN) via a base station CSa as shown in FIG. 6.
FIG. 7 is a diagram showing an operational sequence thereof. Note that in this drawing, the base station CSa is eliminated. Also note that the term “negotiation” as used in FIG. 7 refers to negotiation information.
As well known to those skilled in the art, the PHS architecture offers capability of data communications at a rate of 32 kilobits per second (Kbp) by use of a single TCH which corresponds to the time slot discussed above. The PHS is capable of providing data communication services at 64 Kbps by using a combination or “bundle” of two separate TCHs together. TCH means a time slot channel or a message channel.
Firstly, the mobile station PSa issues to the fixed station FS via the base station CSa a request for interconnection for communication with its destination or target party; then, a communication link is established. Assume in this event that the base station CSa has two idle TCHs, which are assigned by the base station CSa to the “calling” mobile station PSa.
In response to the TCH allocation the mobile station PSa transmits to the fixed station FS a data frame containing therein both negotiation information and specific information indicative of a request for establishment of synchronization of 64-Kbps data communication in a way that complies with the synchronization establishment procedure under a selected data communication protocol (negotiation+synchronization request [64K] 701).
A typical structure of the data frame is shown in FIG. 2, wherein this frame consists essentially of a control information region, user data region, and error detection code region. The control information region is for storage of several data items, including a sequence number for use during re-send controlling operations, effective data length of the user data region, frame type (identifying whether the frame of interest is for use in setting synchronization or in exchanging parameters), and others.
On the other hand the fixed station FS waits for arrival of the data frame as sent from the mobile station PSa in accordance with the data communication protocol's synchronization establishment procedure while alternately changing or switching the data transfer rate between 64 Kbps and 32 Kbps because of the fact that the fixed station FS is not aware of which of the transmitting rates is presently used for the data frame to arrive at the fixed station FS.
After elapse of certain time, the fixed station FS may receive the data frame. Upon receipt of this frame, the fixed station FS transmits, as a response to the 64-Kbps data communication service request contained in this data frame, information that indicates the acceptability of such request toward the mobile station PSa along with negotiation information (negotiation synchronization reception [64K] 702).
Thereafter, the mobile station PSa and fixed station FS exchange communication parameters therebetween (communication parameter setting request [64K], communication parameter setting reception [64K] 703); then, 64-Kbps data communication including “error re-send” procedure (known as 64K-ARQ data communication) gets started (64K_ARQ Synchronization State 704). ARQ means automatic request for repetition.
As time elapsed, a new communication request is issued by a third party to the base station CSa. When this is done, the base station CSa acts to adjust the TCH allocation number with respect to the mobile station PSa so that it decreases to one (1) (synchronization request [32K] 705). In responding thereto, the mobile station PSa newly transmits toward the fixed station FS via the base station CSa an updated data frame that contains therein information indicative of a request for establishment of synchronization of 32-Kbps data communication.
In such event the fixed station FS must experience failure of synchronization due to the fact that the rate of transmission of the data frame from the mobile station PSa has been switched to 32 Kbps, which would result in creation of an FCS continuity error (706). In case this error continues for a predetermined time period or longer, the fixed station FS operates to switch the communication rate to 32 Kpbs for receipt of an incoming data frame.
Whereby, the fixed station FS receives the 32-Kbps data communication synchronization establishing request from the mobile station PSa; then, it sends a burst of information to the mobile station PSa as a response to this request, which information indicates the acceptability of the 32-Kbps data communication request (synchronization reception [32K]707).
Thereafter, the mobile station PSa and fixed station FS exchange communication parameters therebetween (communication parameter setting request [64K], communication parameter setting reception [64K] 708), and then initiate the intended 32-Kbps data communication including error re-send procedure (known as 32K-ARQ data communication) (32K ARQ synchronization state 709).
Upon termination of the third party's communication service, an idle TCH occurs at the base station CSa; at this time, the base station CSa increases the number of usable TCHs allocated to the mobile station PSa up to two (2). In responding, the mobile station PSa again passes a data frame to the fixed station FS by a way of the base station CSa, which frame includes information indicative of a 64-Kbps data communication synchronization establishment request (synchronization request [64K] 710).
When this is done, the fixed station FS experiences failure of synchronization due to the fact that the rate of transmission from the mobile station PSa has been switched to 64 Kbps, which can result in occurrence of an FCS continuity error (711).
In case this error continues for more than a predetermined time interval, the fixed station FS behaves to switch the communication rate up to 64 Kpbs for receipt of the 64-Kbps data communication synchronization establishment request from the mobile station Psa; then, it sends to the mobile station PSa the information indicating that the 64-Kbps data communication request is acceptable (synchronization reception [64K] 712).
Thereafter, the mobile station PSa and fixed station FS exchange communication parameters therebetween (communication parameter setting request [64K], communication parameter setting reception [64K] 713), letting the 64-Kbps data communication including “error re-send” procedure (known as 64K-ARQ data communication)(64K_ARQ Synchronization state 714) get started again.
However, in case the destination or associative party of the mobile station PSa under data communication is a mobile station PSb via a base station CSb as shown in FIG. 6, when any one of the mobile station PSa and mobile station PSb increases in number of TCHs as presently allocated thereto, the mobile station with such updated TCH number attempts unilaterally to make a request for synchronization establishment of the communication rate pursuant to such new assignment number. In this case, if the remaining mobile station is incapable of operating at the updated communication rate due to its shortage of TCHs being presently assigned thereto, then the above-noted synchronization establishment request becomes invalid forcing it to again perform the communication rate synchronization establishment control that has been executed prior to issuance of the request, which can result in waste of time.
Another problem faced with the related art approach is that certain non-negligible time must be consumed every time when switching the communication rate because of the fact that currently available mobile stations are inherently designed to reject entry of any other communication rate synchronization establishment requests before an FCS continuity error actually has continued for a prespecified length of time period.
In related art telecommunications systems including PHS services, these suffer from an extensive increase in requisite processing time whenever the communication rate is to be changed. In addition, the related art systems are encountered with another problem that a caller's terminal can fail to newly establish its intended synchronization during communication with its target or associated party's terminal depending upon circumstances at the party, which in turn leads to risks of recurrent execution of unnecessary control routines.